The ideal image guidance strategy in radiation therapy is to have real-time volumetric and position information of the tumor and surrounding healthy tissue during the treatment itself. One compelling approach is to use magnetic resonance imaging (MRI), which is a non-invasive technique that not only allows real time volumetric imaging but also provides exquisite soft tissue contrast to differentiate cancerous from healthy tissue. To date two base MRI-linac configurations were proposed, i.e. the in-line and the perpendicular configurations, which are defined by the relative orientation of the medical linac with respect to the main magnetic field of the MRI scanner. Regardless the configuration the relative position between the linac and the MRI isocenter is fixed with the linac pointing at it. This fact limits the use of the medical linac to gating or dynamic multileaf collimator (DMLC), or a combination of these two radiation treatment modalities.
What is needed is an electron gun having a geometry capable to robustly function in the presence of high strength external magnetic field for axisymmetric configurations. What is further needed is an electron gun having a geometry where an MRI-linac can operate without the need to isolate the linac using a magnetic shield, where such a configuration would not only leave the magnet homogeneity unchanged but also provide the linac the flexibility to move along the magnet axis of symmetry if the source to target distance needs to be adjusted.